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AP Biology Chapter 9 Griffith 1928 • Proved transformation of bacteria into a mouse • Had two strains of bacteria – An avirulent or nonlethal strain (R) – A virulent of lethal strain (S) Griffith’s Experiment • When he put the virulent strain in the mouse, it died • When he put the avirulent strain in the mouse, the mouse lived! • Then he heated the virulent strain and then put it into the mouse and the mouse lived! • When he put the heated virulent strain + the nonvirulent strain into the mouse, the mouse died • Why? Explanation • Transformation had occurred • The nonvirulent bacteria took up something in the dead heated virulent strain: a “transforming principle” and changed the nonvirulent strain into a virulent strain!! Other Scientists of interest • Before 1940, biologists thought that proteins were the information molecules because they were so complex and had a lot of variety • Avery, Macleod and McCarty in 1944 proved that the transformation principle in Griffith’s experiment was DNA! Other Scientists of interest • Hershey and Chase in 1952 proved that bacteriophages (viruses that attack bacteria) inject DNA into bacterial cells • Franklin and Wilkins used x ray diffraction on DNA to determine the distances between molecules • Watson and Crick in 1953 came up with the model of DNA Chargaff’s rules • He found a simple relationship in DNA called the base pairing rules – Adenine = Thymine – Guanine = Cytosine Watson and Crick • Crick: English phage geneticist at the Cavendish labs at Cambridge University, London England • Watson: American postdoc in Crick’s lab • Both visited Wilkins & Franklin routinely 1951-53 • Derived the overall concept of the chemical relationship • Considered how Chargaff’s rules represented the structure of DNA • Franklin’s X ray data • Built little tin models of the nucleotides and put the DNA model together like a TinkerToy set • Correctly deduced the structure of DNA (double helix) The Double Helix • This is the Watson and Crick model worked out in 1953 and published in a single-page article in Nature of that year. • Was convincing structurally: gave evidence for how DNA replicated • Most famous biology paper ever written! DNA Structure • • Called deoxyribonucleic acid Made up of nucleotides which have 3 parts 1. sugar – deoxyribose 2. Phosphate 3. Nitrogen base Deoxyribose • Pentose sugar = 5 carbons • Carbons on the sugar are numbered 1 through 5 and the first carbon (1’) is linked to one of the four nitrogen bases (ACTG) phosphate • Is attached to the 5’ and 3’ carbon, making a phosphodiester linkage • Forms the sugar phospate backbone of DNA (or the ladder) Nitrogen base • Remember these are connected to the 1’ carbon of the sugar • 2 groups – Purines • Have two ring structures • Adenine and guanine – Pyrimidines • Have one ring structure • Thymine and cytosine • The number of purines = number of pyrimidines DNA molecule • Consists of 2 polynucleotide chains arranged in a coiled double helix • Helix is like a ladder • Two strands run in opposite directions and are said to be antiparallel to each other – The nitrogen bases are bonded by hydrogen bonds – A = T and C=G according to Chargaff’s rules Hydrogen bonds What is the complement of 3’AGCTAC5’? How Does DNA Replicate? • Several research groups worked on this. We’ll discuss one • 1957: Matthew Meselsohn and Fred Stahl • They had 3 hypotheses 1. DNA replication is “semiconservative” – One old strand kept with each of the new molecules; one old paired with one new strand 2. DNA replication is “conservative” – Double strand maintained intact; new strands are together in the new molecule 3. DNA replication is “dispersive” – Strands cut up and the old and new DNA interspersed in both new strands Semiconservative is correct! • Each strand acts as a template for the other, and so the mutation will propagate through successive generations. How Does Replication Start? • The replication complex binds at the origin of replication, which is identified by a particular base sequence. This is initiated by RNA primer • Helicase unwinds the DNA, which is held open with helix-destabilizing proteins. Replication starts in the Y-shaped replication fork. Replication Proceeds on Two Strands • Nucleotides are always added to the 3’ end by DNA polymerase, thus moving in the 3’ to 5’ direction • but the new strands elongate in opposite directions • The leading strand elongates into the fork • The lagging strand elongates away from the fork • Elongation proceeds smoothly on the leading strand Leading and Lagging Strands • As the fork grows, both new strands elongate further • Subunit addition to the lagging strand is by 1002000 base Okazaki fragments. • The lagging strand grows in a discontinuous manner because of the size of the Okazaki fragments • That’s why it lags The Lagging Strand • Notice that the lagging strand is always growing away from the replication fork • The gaps between the Okazaki fragments are joined together by DNA ligase In Action! Review of Enzymes • • • • • Unwinds the DNA Puts down RNA primer Adds bases to strands Seals Okazaki fragments up Winds the DNA molecule back together DNA Repair • DNA polymerase proofreads what bases had been laid down • If there is a mistake, it will go back and remove the wrong base and fix it How does DNA fit in the cell? • By histones – Positively charged proteins (due to the high number of amino acids) – Are able to associate with DNA which is negatively charged (due to the phosphate groups) • Histones and DNA form structure called nucleosome Nucleosome • Are 8 histones with DNA wrapped around it • Are part of the chromatin • Prevent DNA strands from becoming tangled Cellular Ageing and DNA • The replication process never entirely completes at the ends of the chromosomes • However, DNA is protected at its ends with long strands that do not carry any genetic information, called telomeres • as we age, they become shorter • They are repaired and lengthened with an enzyme called telomerase • Loss of telomerase activity may be an important cause of cellular aging